Soft resins such as olefin copolymer rubber like ethylene/propylene copolymer rubber, and ethylene/propylene/diene copolymer rubber are marketed usually in a shape of blocks such as bales.
Such a block-shaped olefin copolymer rubber have disadvantages of troublesomeness in taking-out from a storage site, and in supply or transportation to a molding and processing apparatus, and in weighing.
Accordingly, the olefin copolymer rubber, if supplied in a pellet form, can be free from the above disadvantages, and has various advantages: an extruder of high productivity can be used for preparing a compounded rubber from the pelletized olefin copolymer rubber and a compounding ingredient in place of the less efficient mixer like a Banbury mixer conventionally used for blending an olefin copolymer rubber and a conventional formulating ingredient such as fillers, softeners, and vulcanizing agents.
However, the above olefin copolymer rubber, even if it is pelletized, will cohere together into blocks during storage owing to the stickiness of the olefin copolymer resin itself to lose the advantages of the pelletization.
Even an ethylene/α-olefin copolymer elastomer which is less sticking at an ordinary temperature can cause cohesion of the pellets when the pellets are stored with a load applied thereon or stored in a high temperature environment in summer, thereby decreasing the value as a pelletized product.
Various methods have been disclosed for decreasing the surface stickiness of the pellets of the inherently sticky olefin copolymer rubber, or preventing cohesion of the pellets of an olefin elastomer, which is less sticky in ordinary temperature, under a load and/or at a high temperature. For example, the surface of the rubber pellets is coated with a silicone oil (Japanese Patent Application Laid-Open No. 48(1973)-47934). In another method, a powdery matter such as powdery inorganic substances like talc, silica, and calcium carbonate, or a powdery polyethylene is dusted onto the pellets as an anti-blocking agent to allow the powdery matter to adhere to the sticky pellet surface.
However, the coating of the surface of the rubber pellets or elastomer pellets with a silicone oil has a disadvantage that the intended sufficient effects cannot be achieved for the pellets having a strong stickiness. With the latter method, the powdery matter dusted can give adverse effects in the properties of the rubber in end use, since the inorganic material like talc and silica is not compatible with the olefin copolymer rubber. Furthermore, in this method of dusting of a powdery matter, the stickiness-preventing powdery matter is used in an amount from several to ten-and-several percents based on the weight of the pellets to prevent substantially the cohesion of the pellets. Such a large amount of the powdery matter will impair the properties of the treated olefin copolymer rubber. For example, the rubber properties of a vulcanized olefin copolymer rubber dusted with powdery polyethylene become deteriorated with increase of the amount of the adhering powdery polyethylene. Further, with this method, dustiness of the pellet surface impairs the external appearance of the pellets and makes the handling troublesome.
An effective countermeasure against the pellet cohesion is disclosed by the inventors of the present invention (Japanese Patent Application Laid-Open No. 56(1981)-136347) in which rubber pellets are coated with a higher fatty acid and/or a salt thereof. This method can prevent the sticking of rubber pellets without impairing the rubber properties substantially.
However, the inventors of the present invention found that the method disclosed in the above Patent Laid-Open Publication cannot allow the higher fatty acid and/or its salt to adhere stably in a sufficient amount onto rubber pellets since the higher fatty acid and/or its salt is simply mixed with the rubber, and may still cause blocking (cohesion) during storage, or after transportation, weighing, and other treatment.
The inventors of the present invention, after comprehensive investigation, disclosed a method of preventing the blocking of the rubber pellets during storage, in which the olefin copolymer rubber pellets, fine powdery higher fatty acid of 12–30 carbon atoms and/or its salt, are mixed in the presence of a monohydric alcohol of 1–4 carbon atoms to prepare less sticky rubber pellets (Japanese Patent Publication No. 4(1992)-1011). The rubber pellets coated with the higher fatty acid and/or its salt produced according to this method do not cause blocking, even after the pelletized rubber is transported, packed into bags in an amount of 25 kg per bag, and bags are stacked up in 10 stairs for one month. In this method, however, the powder-dusted state of the pellet surface may cause some trouble in handling.
At the moment, to meet the users' request, appearance of a pelletized soft resin which is nonsticky and has excellent appearance, and is easily handleable in comparison with conventional pelletized soft resins like olefin copolymer rubbers, and a method of producing the same is desired.
The present invention intends to solve the above problems of the prior art techniques, and to provide a soft resin pellet which is nonsticky and has excellent appearance, and is easily handleable in comparison with conventional pelletized soft resins like olefin copolymer rubbers, and a process for production thereof.